1. Field of the Invention
The present invention relates to strain and crack sensors and, more specifically, to a wireless passive strain and crack sensing systems.
2. Description of the Related Art
In order to accurately assess deterioration of civil, mechanical, and aerospace structures, there has been a large volume of research in structural health monitoring (SHM) over past few decades. Sensors can be used to measure various structural responses and operating conditions, including: strain, displacement, acceleration, humidity, temperature, etc. Among these measurements, strain can be an important indicator for stress concentration and damage development.
Metal foil strain gages are currently among the most common solutions due to their low-cost, simple circuitry, and acceptable reliability in many applications. However, when applied to large structures, traditional metal foil strain gages require lengthy cable connections for power and data acquisition, which can significantly increase installation time and system cost.
Wireless strain sensors have recently been developed to avoid cabling difficulty associated with metal foil strain gages. For example, one wireless strain sensor employs the inductive coupling principle involving two adjacent inductors. However, the interrogation distance achieved by inductive coupling is usually limited to several inches, which is inconvenient for many practical applications. Electromagnetic backscattering techniques have been exploited for wireless strain sensing in an attempt to increase interrogation distance.
Since the electromagnetic resonance frequency of a planar antenna is related to the antenna's physical dimension, the resonance frequency changes when the antenna is under strain. This relationship between resonance frequency and strain can be used for stress/strain measurement of a structure to which the planar antenna is bonded. For example, a patch antenna has been designed for wireless strain sensing in which a phototransistor is adapted for signal modulation of the RF signal backscattered from the antenna sensor. As a result, signal backscattered from the sensor can be distinguished from environmental reflections. However, the light-switching mechanism is not practical for outdoor application, where light intensity is usually so strong that the phototransistor is constantly activated and thus, loses the ability to switch.
To avoid this difficulty, a low-cost off-the-shelf radiofrequency identification (RFID) chip has been previously adopted as a simple mechanism for signal modulation. Since the RFID chip is powered by a wireless interrogation signal, the RFID-based strain sensor is wireless and passive (i.e., battery-free). One prototype RFID antenna sensor has shown a strain measurement resolution of 20μ∈ in laboratory experiments, and can measure large strains up to 10,000μ∈. Previous studies demonstrated that if operating frequency of the wireless strain sensor is increased, strain sensitivity can be improved and sensor size can be reduced. However, the RFID chip only functions in the frequency band of 860-960 MHz.
Therefore, there is a need for a wireless passive strain sensor that is configured to operate at frequencies higher than typical RFID frequencies.